Intermetallic compounds

ABSTRACT

A method for the production of an intermetallic compound (M 1 Z) involves treating a solid precursor material comprising three or more species, including first and second metal or metalloid species (M 1 , Z) and a non-metal species (X), by electro-deoxidation in contact with a melt comprising a fused salt (M 2 Y) under conditions whereby the non-metal species dissolves in the melt. The first and second metal or metalloid species form an intermetallic compound. The method is performed in a cell comprising a cathode of the precursor material ( 2 ), which is immersed in a melt ( 8 ) contained in a crucible ( 6 ) for electro-deoxidation.

FIELD OF THE INVENTION

[0001] This invention relates to a method and an apparatus s forpreparing intermetallic compounds, and to intermetallic compounds soproduced.

BACKGROUND TO THE INVENTION

[0002] Intermetallic compounds are compounds of a defined structurecomprising a metal and either a non-metal (metalloid) or further metal,They have many applications. For example silicon carbide is used inmetal matrix composites as a strengthening additive and for furnaceelectrodes. Molybdenum silicide is also used as a furnace element and asa strengthening agent. Titanium diboride is used as a possible cathodematerial for the Hall-Heroult cell for the extraction of alumina.

[0003] Carbides are amongst the most refractory materials known. Manycarbides have softening points above 3000° C. and the more refractorycarbides possess some of the highest melting points ever measured. Ofthe simple carbides, the most refractory are HfC and TaC, which melt at3887° C. and 3877° C. The complex carbides 4TaC.ZrC and 4TaC.HfC melt at3932° C. and 3942° C., respectively. Silicon carbide is quite resistantto oxidation at temperatures up to about 1500° C and has usefuloxidation resistance for many purposes at temperatures up to 1600° C. Itis used extensively for example as an abrasive, as a refractory and as aresistor element for electric furnaces.

[0004] Most carbides have fair thermal and electrical conductivity, andmany of them are quite hard, boron carbide being the hardest. Highhardness accounts for the usefulness of many of the carbides, such assilicon carbide, titanium carbide, boron carbide and tungsten carbide asmaterials for cutting, grinding and polishing and for parts subject tosevere abrasion or wear.

[0005] Most carbides are prepared by the reaction of the oxide withcarbon at elevated temperatures. Other methods of preparation includevapour deposition from the gaseous phase.

[0006] The carbides of Group II elements are usually preparedcommercially by reacting the oxide with graphite in an electric-arcfurnace at around 2000° C. Boron carbide and silicon carbide are made bya similar route, as are transition or hard metal carbides. High puritycarbides are difficult to prepare commercially.

[0007] TiB₂ and ZrB, have potential for replacing carbon as an electrodematerial in aggressive electrochemical applications such as aluminiumrefining. Their good electrical conductivity, good wettability andexcellent chemical resistance means greatly increased lifetimes. TiB₂ isharder than tungsten carbide and has an excellent stiffness to weightratio so it has important applications for cutting tools, crucibles andother corrosion resistance applications.

[0008] Boride powders can be prepared by the carbothermic oraluminothermic reduction of metal oxide-boron oxide mixtures, byelectrolysis of fused salt mixtures containing metal oxides and boronoxide and by heating mixtures of metal and boron powders to hightemperatures in an inert atmosphere. Fusion electrolysis is especiallysuited to the large-scale production of boride powders of relativelyhigh purity from naturally occurring raw materials, and does not requirethe initial preparation of metal and boron powders. However, the currentefficiency is very low of the order of 5%.

[0009] Of conventional methods, direct synthesis of refractory boridespermits the greatest control of composition and purity of the resultingboride. However, the temperature required is very high (1700° C.).

[0010] Conventionally, silicides can be prepared by six general methods,i.e. synthesis from the elements (metal and silicon); reaction of metaloxide with silicon;

[0011] reaction of metal oxide with silicon and carbon; and

[0012] reaction of silica and metal oxide with carbon, aluminium ormagnesium. The silicides are chemically inert, have s high thermal andelectrical conductivities, are hard and have high strengths at elevatedtemperatures coupled with high melting points.

[0013] Aluminides are made by the direct reaction of the elements.

[0014] Generally, these interesting materials are made at very hightemperatures where it is difficult to ensure high purity. Theelectrochemical methods that have been tried generally work at very lowcurrent efficiencies.

SUMMARY OF THE INVENTION

[0015] The invention provides a method and an apparatus for, makingintermetallic compounds, and the intermetallic compounds so produced, asdefined in the appended independent claims. Preferred or advantageousfeatures of the invention are set out in dependent subclaims.

[0016] The present invention is based on the surprising finding thatintermetallic compounds can be made using a simple electrochemicalprocess. Thus, the invention may advantageously provide a method for theproduction of an intermetallic compound (M¹Z) which involves treating asolid precursor material comprising three or more species, each speciesbeing for example an element or an ion, or other component of a compoundsuch as a covalent compound. The three or more species include first andsecond metal or metalloid species (M¹,Z) and an anionic or non-metalspecies (X), and the precursor material is treated byelectro-deoxidation in contact with a melt comprising a fused salt (M²Y)under conditions whereby the anionic or non-metal species dissolves inthe melt. The first and second metal or metalloid species then form anintermetallic compound. More complex intermetallic compounds comprisingthree or more metal or metalloid species may similarly be formed. In theprecursor material, the metal or metalloid species may advantageously bepresent in the appropriate ratios to form a stoichiometric intermetallicwith minimum wastage.

[0017] In one embodiment, the precursor material may consist of a singlecompound. For example, if the precursor material is formed of titaniumborate powder, then the first and second metals or metalloids, Ti and B,can form TiB₂ when the anionic or non-metal species, O²⁻, is removed byelectro-deoxidation. Corresponding results may be achieved by usingprecursor materials comprising other ions such as CO₃, SO₄, NO₂ or NO₃in which both a metal or metalloid species and an anionic or non-metalspecies are present.

[0018] In an alternative embodiment the precursor material may comprisea compound such as those described above mixed with a further substance,such as a further compound or an element or a more complex mixture,which may advantageously enable the formation of more complexintermetallics.

[0019] In another embodiment, the precursor material may be a mixture ofa first solid compound (M¹X) between the first metal or metalloid (M¹)and the anionic or non-metal species (X), and a solid substance (S)which consists or comprises the second metal or metalloid (Z). In thiscase, the substance (S) may be an element (i.e. the metal or metalloid(Z) itself) or an alloy, or it may be a second compound comprising thesecond metal or metalloid (Z) and a second anionic or non-metal species.Advantageously, the second non-metal species may then be the same as thenon-metal species (X) in the first compound (M¹X).

[0020] The term electro-deoxidation is used herein to describe theprocess of removing the anionic or non-metal species (X) from a compoundin the solid state by contacting the compound with the melt and applyinga cathodic voltage to the compound(s) such that the non-metal speciesdissolves or moves through the melt to the anode. In electrochemistry,the term oxidation implies a change in oxidation state and notnecessarily a reaction with oxygen. It should not, however, be inferredthat electro-deoxidation always involves a change in the oxidationstates of the components of the compound; this is believed to depend onthe nature of the compound, such as whether it is primarily ionic orcovalent. In addition, it should not be inferred thatelectro-deoxidation can only be applied to an oxide; any compound may beprocessed in this way.

[0021] In a preferred embodiment, the cathodic voltage applied to themetal compound is less than the voltage for deposition of cations fromthe fused salt at the cathode surface. This may advantageously reducecontamination of the intermetallic compound by the cations. It isbelieved, that this may be achieved under the conditions of anembodiment providing a method for the production of an intermetalliccompound (M¹Z) comprising treating a mixture of a metal compound (M¹X)and a substance (Z) by electrolysis, or electro-deoxidation, in a fusedsalt (M²Y), under conditions whereby reaction of X rather than M²deposition occurs at an electrode surface, and X dissolves in theelectrolyte M²Y, or moves through the melt to the anode. In variousinstances, the process of electro-deoxidation may alternatively betermed electro-decomposition, electro-reduction or solid-stateelectrolysis.

[0022] Further details of the electro-deoxidation process are set out inInternational patent application number PCT/GB99/01781, which isincorporated herein by reference in its entirety.

[0023] The precursor material is advantageously formed by powderprocessing techniques, such as compaction, slip-casting, firing orsintering, from its constituent material or materials in powder form.Preferably the precursor material so formed is porous, to enhancecontact with the melt during electro-deoxidation. The precursor materialmay alternatively be used in the form of a powder, suitably supported orpositioned in the melt.

[0024] Advantageously, if the precursor material is a conductor it maybe used as the cathode. If C or B powder is incorporated to formcarbides or borides, this will generally increase the conductivity ofthe mixture. Alternatively, the precursor material may be an insulatorand may then be used in contact with a conductor.

[0025] In the method of invention, it is preferable for theintermetallic compound produced to have a higher melting point than thatof the melt.

[0026] The method of the invention may advantageously give a productwhich is of very uniform particle size and free of oxygen or othernon-metal species from the precursor material.

[0027] A preferred embodiment of the present invention is based on theelectrochemical reduction of an oxide powder in combination with afurther metal, non-metal (metalloid) or compound (which may be in theoxide form), by cathodically ionising the oxygen away from the oxide sothat the reduced substances combine together to form intermetalliccompounds. Thus, in a preferred embodiment, the method for making theintermetallic compounds relies on making a mixture of oxide powders thecathode in a melt comprising a fused salt, such that the ionisation ofoxygen takes place preferentially rather than the deposition of cationsfrom the salt, and that the oxygen ions are mobile in the melt.

SPECIFIC EMBODIMENTS AND BEST MODE OF THE INVENTION

[0028] Embodiments of the invention will now be described by way ofexample, with reference to the accompanying drawings, in which;

[0029]FIG. 1 illustrates an apparatus according to a first embodiment ofthe invention;

[0030]FIG. 2 illustrates an apparatus according to a second embodimentof the invention; and

[0031]FIG. 3 illustrates an apparatus according to a third embodiment ofthe invention.

[0032]FIG. 1 shows two pellets 2 of a precursor material, which in thiscase is a mixture of metal oxides, in contact with a cathode conductor4, such as a Kanthal wire. Each pellet is prepared by pressing orslip-casting micrometre-sized powders (for example up to about 25 μm or100 μm, or between about 0.2 and 2 μm particle size) and then, usually,firing or sintering. This produces a porous pellet, which advantageouslyallows intimate contact between the precursor material and the meltduring electro-deoxidation. The pellet is then made the cathode in acell comprising an inert crucible 6, such as an alumina or graphitecrucible, containing a fused salt 8. On the application of current(making the pellets the cathode), the oxygen in the metal oxides ionisesand dissolves in the salt, and diffuses to a graphite anode 10, where itis discharged. Effectively the oxygen is removed from the oxides,leaving the metals behind.

[0033] The electrolyte, or melt, 8 consists of a salt or salts which arepreferably more stable than the equivalent salts of the individualelements of the intermetallic compound which is being produced. Morepreferably, the salt should be as stable as possible to remove theoxygen to as low a concentration as possible. The choice includes thechloride, fluoride or sulphate salts of barium, calcium, cesium,lithium, strontium and yttrium or even Mg, Na, K, Yb, Pr, Nd, La and Ce.

[0034] To obtain a salt with a lower melting point than that given by apure salt, a mixture of salts can be used, preferably the eutecticcomposition. In the embodiment, the cell contains chloride salts, beingeither CaCl₂ or BaCl₂ or their eutectic mixture with each other or withanother chloride salt such as NaCl.

[0035] At the end of reduction, or electro-deoxidation, the reducedcompact, or pellet, is withdrawn together with the salt contained withinit. The pellet is porous and the salt contained within its poresadvantageously stops it from oxidising. Normally, the salt can simply beremoved by washing in water. Some more reactive products may need to becooled first in air or in an inert atmosphere and a solvent other thanwater may be required. Generally, the pellets are very brittle and caneasily be crushed to intermetallic powder.

[0036]FIG. 2 shows an apparatus similar to that of FIG. 1 (using thesame reference numbers where appropriate) but using a conductivecrucible 12 of graphite or titanium. The pellets sink in the melt andcontact the crucible, to which the cathodic voltage is applied. Thecrucible itself thus acts as a current collector.

[0037]FIG. 3 shows an apparatus similar to that of FIGS. 1 and 2 (usingthe same reference numbers where appropriate) but in which the precursormaterial is supported in a smaller crucible 14 which can be lowered andraised into and out of the melt, suspended on a wire 16 which alsoallows electrical connection so that the smaller crucible, which iselectrically conducting, can act as a cathodic current collector. Thisapparatus may advantageously be more flexible than that of FIG. 1 or 2in that it may be used for electro-deoxidation not only of pellets orthe like but also of loose powders or other forms of precursor material18.

[0038] In a further embodiment, the smaller crucible may be inverted toallow treatment of precursor materials less dense than the melt. Aninverted smaller crucible may be covered by a grid to retain materialson immersion into and removal from the melt. The smaller crucible mayeven be closed, apart from apertures to allow access by the melt, forbetter retention of the precursor material and the reaction product.

[0039] The following Examples illustrate the invention.

EXAMPLE 1

[0040] A pellet, 5 mm in diameter and 1 mm in thickness was formed froma mixture of SiO₂ and C powders, and placed in a carbon crucible filledwith molten calcium chloride at 950° C. A potential of 3 V was appliedbetween a graphite anode and the graphite crucible (as in FIG. 2). After5 hours, the pellet was removed from the crucible, the salt allowed tosolidify and then dissolved in water to reveal the intermetalliccompound.

[0041] The cathodic reaction is SiO₂+C+4e=SiC+20²⁻

EXAMPLE 2

[0042] A pellet, 5 mm in diameter and 1 mm in thickness, of titaniumdioxide powder and boron powder or, in a separate test, a pellet formedof titanium borate powder was placed, in a crucible containing moltenbarium chloride at 950° C. A potential of 3 V was applied between agraphite anode and the crucible. After 5 hours, the pellet was removedfrom the crucible, the salt allowed to solidify and then dissolved inwater.

[0043] The cathodic reaction that had occurred was

TiO₂+2B+4e=TiB₂+20²⁻

or

TiO₂.B₂O₃+10e=TiB₂+50²⁻

EXAMPLE 3

[0044] A pellet, 5 mm in diameter and 1 mm in thickness, of mixedpowders of molybdenum oxide and silicon or, in a separate test,molybdenum oxide and silicon dioxide was placed in a graphite cruciblefilled with molten calcium chloride at 950° C. A potential of 3 V wasapplied between a graphite anode and the graphite crucible. After 5hours, the pellet was removed from the crucible, the salt allowed tosolidify and then dissolved in water.

[0045] The reaction which had taken place was

MoO₂+2Si+4e=MoSi₂+20²⁻

or

MoO₂+2SiO₂+12e=MoSi₂+60²⁻

EXAMPLE 4

[0046] A pellet, 5 mm in diameter and 1 mm in thickness, of mixedpowders of alumina and titanium dioxide was placed in a titaniumcrucible filled with molten calcium chloride at 950° C. A potential of 3V was applied between a graphite anode and the titanium crucible. After5 hours, the pellet was removed from the crucible, the salt allowed tosolidify and then dissolved in water.

[0047] The reaction which had taken place at the cathode was

Al₂O₃+2TiO₂+14e=2TiAl+70²⁻

[0048] It can be appreciated that, by varying the ratio of theconstituents, the ratios of the elements in the intermetallic compoundcan be varied.

EXAMPLE 5

[0049] Molybdenum disilicide. Powders of MoO₃ and SiO₂ were, mixedtogether, pressed into a pellet and sintered at 600° C. The sinteredpellet was put into a steel crucible and lowered into a larger containerof molten calcium chloride at 785° C. A voltage of 3.0 V was applied for24 hours between the pellet and a graphite anode. The crucible wasremoved from the melt and washed with water. After filtering and dryingthe powder it was analysed by XRD (X-ray diffraction) which revealed anabundance of MoSi₂ with a smaller quantity of other compounds such asCaSiO₃, CaCO₃ and SiC.

EXAMPLE 6

[0050] The above experiment was repeated with a MoO₃/SiO₂ mixturesintered at 650° C. After reducing the pellet for 24 hours at 3.0 V thecrucible containing the pellet was washed with distilled water and thenwith 0.1 M hydrochloric acid. XRD of the remaining powder againconfirmed the production of MoSi₂ but CaSiO₃ and SiC remained as minorconstituents.

EXAMPLE 7

[0051] Titanium carbide. TiO₂ and graphite powders were mixed andpressed into pellets which were sintered for 1 hour at 1200° C. in avacuum furnace. These pellets were placed in a small alloy steelcrucible which was then immersed in calcium chloride at 800° C. for 43hours using 3.0 V. When the small crucible was removed from the melt andwashed in water a black powder remained. EDX (energy-dispersive X-rayanalysis) and XRD analysis of the filtered and dried fine powderconfirmed the production of TiC.

EXAMPLE 8

[0052] Zirconium carbide. ZrO₂ and graphite powders were mixed andpressed into pellets. The pellets were sintered at 1200° C. for 1 hourin a vacuum furnace. The pellets were reduced in molten calcium chlorideat 800° C. for 43 hours using 3.0 V. After washing in water for 2 days,filtering and drying, the remaining powder and lumps were ground andanalysed by XRD. ZrC was clearly the dominant compound with a littleCaZrO₃ and carbon also present. EDX confirmed that Zr and C were thedominant elements.

EXAMPLE 9

[0053] Tantalum carbide. Ta₂O, and graphite powders were mixed andpressed into pellets and sintered in a vacuum furnace at 1200° C. for 1hour. The pellets were then reduced in calcium chloride at 800° C. using3.0 V for 25 hours. XRD analysis of the powder confirmed TaC with a verysmall amount of Ta also present. EDX analysis confirmed the high purityof the product.

EXAMPLE 10

[0054] Titanium diboride. TiO₂ and boron powders were mixed and pressedinto pellets which were sintered for 1 hour at 1200° C. in a vacuumfurnace. These pellets were then reduced for 24 hours at 800° C. using3.0 V. EDX and XRD analysis of the resulting fine powder confirmed theproduction of TiB₂.

EXAMPLE 11

[0055] Zirconium diboride. ZrO₂ (yttria stabilised) and boron powderswere mixed and pressed into pellets before sintering at 1200° C. for 1hour in a vacuum furnace. The pellets were then reduced in a calciumchloride melt at 800° C. using 3.0 V for 25 hours. XRD of the resultingpowder and lumps revealed ZrB₂ and Y₂O₃ with no other compound beingdetected. The high purity of the product and the fact that the yttriaremained unreduced while the zirconia was completely converted to theboride is a significant result. EDX analysis indicated about 2% calciumwhich was not apparent on the XRD result.

EXAMPLE 12

[0056] Chrome silicon. SiO₂ and Cr₂O₃ powders were mixed and formed intopellets which were sintered in air. The pellets were reduced in a moltenmixture consisting of about 85% sodium chloride and 15% calcium chlorideat 800° C. for 20 hours using 3.0 V. After washing in water and drying,the resulting lumps were ground and analysed by XRD. Cr₃Si, Cr₅Si₃,CaCO₃, CrSi₂, CrSiO₄, and CaSiO₃ were, all present in order ofdecreasing abundance. EDX showed grains about 2 μm diameter containingmainly Si, Cr, Ca and O.

EXAMPLE 13

[0057] Silicon titanium. SiO₂ and TiO₂ powders were mixed and formedinto pellets which were sintered in air. The pellets were reduced in amolten mixture consisting of about 85% sodium chloride and 15% calciumchloride at 800° C. for 19 hours using 3.0 V. After washing in water anddrying the lumps were ground and analysed by XRD. Ti₅Si₃, Ca₂SiO₄,Ti₅Si₄, TiSi and Si were all present in order of decreasing abundance.EDX showed a porous matrix containing mainly Si, Ti, Ca and O.

[0058] Further Aspects and Embodiments

[0059] The need to fire the metal oxide/graphite pellets in a vacuumfurnace in a number of the embodiments described above adds cost to theprocess. Although the temperatures required are advantageously muchlower than when using the conventional direct synthesis route to, forexample, carbide production, an alternative system could be of benefit.If one of the more stable carbonates such as K₂CO₃ or Na₂CO₃ was mixedinto the precursor material the carbonate would be decomposed duringelectrolysis and some of the carbon would react with the other cationsin the precursor to form carbides. Sodium and potassium do not formstable carbides so they would come out of the reactor as the metalitself, which could be removed with alcohol.

[0060] Boron-metal oxide mixed pellets may be sintered in air because avery thin protective boron oxide layer forms and prevents furtheroxidation. However, the use of elemental boron has the disadvantage thatit is not the cheapest source of boron. Boron occurs naturally as boronoxide, sodium borate, and calcium borate. Boron oxide is a glass andsoftens above 500° C. which means that unless it reacts in some way withthe metal oxides or other compounds also making up the pellet it may bedifficult to hold the pellet in or on the cathode. Boron oxide is also,typically less dense than the electrolyte so it will tend to float whilemost metal oxides will tend to sink. The boron oxide may also, becauseof softening at elevated temperatures, form a non-porous pellet whichwould slow the electro-deoxidation. The electrolyte temperature could bereduced to below 450° C. by using a mixture of halide salts, but thatmay add cost and slow the reduction even further.

[0061] Sodium borate has a higher melting point than boron oxide so itis easier to use to make a mixed pellet. Reduction of the pellet maythen advantageously form the desired boride and sodium metal. The sodiummetal could be easily and safely removed from the reduced pellet byimmersing it in methanol or ethanol. Calcium borate has even moreadvantages than sodium borate because its melting point is even higherand the calcium metal by-product can be removed safely with water.

[0062] Silicon very readily combines with calcium to form calciumsilicate as shown by all XRD analyses performed on precursor materialswhich had started with silica in them and were processed in calciumsalts. Much of the silicon may disadvantageously be wasted because ofthis. It has been found, however, that by using a molten electrolytethat contains little or no calcium salts it was possible to reduce thisproblem considerably. For example, sodium chloride or other sodium saltsor salts of other metals such as alkali or alkaline earth metals oryttria may be used.

1. A method for the production of an intermetallic compound (M¹Z)comprising: treating a solid precursor material comprising three or morespecies, including first and second metals or metalloids (M¹,Z) and anon-metal species (X), by electro-deoxidation in contact with a meltcomprising a fused salt (M²Y) under conditions whereby the non-metalspecies dissolves in the melt.
 2. A method according to claim 1, inwhich the precursor material consists of a single compound.
 3. A methodaccording to claim 1, in which the precursor material is a mixture of afirst solid compound (M¹X) comprising the first metal or metalloid (M¹)and the non-metal species (X), and a solid substance (S) which consistsof or comprises the second metal or metalloid (Z):
 4. A method accordingto claim 3, in which the substance (S) is a second compound, comprisingthe second metal or metalloid (Z) and a second non-metal species.
 5. Amethod according to claim 4, in which the second non-metal species isthe same as the non-metal species (X) in the first compound (M¹X).
 6. Amethod according to any preceding claim, in which the precursor materialis a conductor and is used as the cathode.
 7. A method according to anyof claims 1 to 5, in which the precursor material is an insulator and isused in contact with a conductor.
 8. A method according to any precedingclaim, in which electrolysis is carried out at a temperature of 700°C.-1000° C.
 9. A method according to any preceding claim, in which theelectrolysis product (M²X) is more stable than the precursor material orthe first compound.
 10. A method according to any preceding claim, inwhich the fused salt comprises Ca, Ba, Li, Cs and/or Sr.
 11. A methodaccording to any preceding claim, in which the fused salt comprises Cl,F and/or SO₄.
 12. A method according to any preceding claim, in whichthe non-metal species comprises O, S, C and/or N.
 13. A method accordingto any preceding claim, in which the non-metal species comprises Oand/or S.
 14. A method according to any preceding claim, in which theprecursor material comprises a compound incorporating the anion CO₃,SO₄, NO₂ and/or NO₃.
 15. A method according to any preceding claim, inwhich the first metal or metalloid comprises Ti, Si, Ge, Zr, Hf, Sm, U,Al, Mg, Nd, Mo, Cr or Nb, any other lanthanide or any other actinide.16. A method according to any preceding claim, in which the second metalor metalloid comprises C, B, Si or Al.
 17. A method according to any ofclaims 3 to 16, in which one or both of the first and second compoundsis an oxide.
 18. A method according to any preceding claim, in which theelectro-deoxidation is carried out under conditions whereby the cathodicvoltage applied to the precursor material is less than the voltage fordeposition of a cation (M²) from the fused salt at the cathode surfaceor, if the melt comprises a mixture of salts, less than the voltage fordeposition of any cation (M²) from the melt at the cathode surface. 19.A method for the production of an intermetallic compound (M¹Z)comprising: treating a mixture of a solid compound (M¹X), comprising ametal or metalloid (M¹) and a non-metal species (X), and a solidsubstance (S), by electro-deoxidation in contact with a melt comprisinga fused salt (M²Y) under conditions whereby the non-metal speciesdissolves in the melt.
 20. A method according to claim 19, in which thesubstance (S) is a second compound, comprising a second metal ormetalloid (Z) and a second non-metal species.
 21. A method according toclaim 19, in which the second non-metal species is the same as thenon-metal species (X) in the first compound (M¹X), both non-metalspecies preferably being O.
 22. An intermetallic compound produced usinga method as defined in any preceding claim.
 23. An apparatus forcarrying out a method as defined in any of claims 1 to 21.